Blank Preview When Uploading Images – Post_Thumbnail Issue?

I’m modifying a previous theme for a client and I’m having some trouble when I add theme support for Post_Thumbnails. The issue happens when you try to upload a new image. Normally after an upload completes it will bring up a preview of the image and some options for embedding. For some reason I don’t get any of this now and just get a 4-digit number. Anybody have an idea what may be causing this? Here is a screenshot of the problem.